A stable organic triradical with truncated π-conjugation as a model for single-component organic molecule-based ferrimagnetics

Exchange Interaction in Covalently Bonded Biradical−Monoradical Composite Molecules

“…9,11,12 It has been known that the simulated hyperfine splitting patterns are insensitive to the difference of |J(π)|, |J(σ)|, and |J′(σ)| in the strong exchange limit. 9,11,12 Although the amplitude of J(π) is expected to be much larger than those of J(σ) and J′(σ) from the chemical structure of 4, the deviation of molecular symmetry of exchange interactions from equilateral as well as isosceles triangles is not detectable in hyperfine ESR spectroscopy of fluid solutions for the strong exchange limit. The hyperfine coupling constants for 5 are in agreement with those for nitronyl nitroxide monoradicals with similar structures.…”

“…This gives the relative amplitude of |J(π)|, |J(σ)|, and |J′(σ)|, which has been missed in the hyperfine structure analysis of only 14 N nuclei. We have reported an ESR spectral analysis of nitroxide-based triradicals on the basis of perturbation treatments 11,12 assuming that the exchange interaction within the biradical moiety J(π) is much larger than those between the biradical and monoradical moieties |J(σ)| and |J′(σ)|. This approach is applied to the explanation of the superposition by introducing the 1 H hyperfine coupling terms as well as those of 14 N. To the spin Hamiltonian eq 8 is added the 1 H hyperfine coupling terms and it is split into two parts, H (0) and H (1) , or H (0) and H (2) :…”

“…The amplitude of the intramolecular ferromagnetic interaction J(π) in the biradical moiety is close to those of triradicals with similar molecular structures of the m-phenylene-bis(nitronyl nitroxide) moiety. 9,11 The mean-field θ is regarded as an average of J(σ), J′(σ), J 1 , and J 2 .…”

“…In contrast, intermediate cases of |J| ≈ |A| give rise to complicated hyperfine splittings as compared with monoradicals of S ) 1 / 2 with one unpaired electron. We present here an alternative approach based on perturbation treatments 11,12 assuming that the amplitude of J(π) is much larger than those of |J(σ)| and |J′(σ)|. This approach gives an analytical form of resonance fields of hyperfine transitions due to 14 N and 1 H, and enables us to assign specific signals in complicated hyperfine splitting patterns, affording estimates for the magnitude of weak couplings J(σ) and J′(σ).…”

Hyperfine structure of ESR spectra as a probe for heisenberg exchange couplings in nitroxide triradicals serving as building blocks for molecule-based ferrimagnets

“…ESR spectra of a triradical with three identical nitroxide groups have been analyzed in terms of the relaxation matrix [43,44] and the composite Liouville space formalism [45]. We present here an alternative approach on the basis of perturbation treatments [46] assuming that the exchange interaction within the biradical moiety ]Jl(n)l is much larger than those between the biradical and the monoradical moieties rJ2(~)l, [J3(cr)[. This approach gives the assignment of specific signals in complicated hyperfine splitting patterns, which is difficult to obtain by the use of the exact diagonalization described above, explaining the appearance of the triad signals for 2.…”